Primary mucosal melanomas represent ∼1.3% of all cases of melanoma diagnosed in the USA. The sinonasal location is the most common primary site. Mutations in the KIT gene occur in 10–22% of mucosal melanomas. Tumor response to imatinib mesylate has been reported in about half of the patients with tumors harboring KIT mutations. Responses are almost exclusively restricted to tumors with mutations in KIT exon 9 or 11. We report a case of a patient with a sinonasal mucosal melanoma with a novel exon 8 mutation (C443S) who had marked initial response to imatinib. Somatic exon 8 KIT mutations have not been previously reported in mucosal melanoma or in other human solid tumors; however, such mutations have been reported in canine and feline mast cell tumors. Protein transcripts from exon 8 play an important role in the structural and functional integrity of the extracellular domain of KIT. In preclinical studies, a mutation in exon 8 led to autophosphorylation, independent of KIT ligand, and constitutive activation of the tyrosine kinase. This biology may explain the successful application of imatinib in animals with tumors harboring exon 8 KIT mutations and in our patient with mucosal melanoma. This report expands the population of patients with melanoma who might benefit from imatinib to those with somatic exon 8 KIT mutations. Such mutations should be looked for in patients with mucosal melanoma.